The present invention relates to graphic display apparatus having a writing pad, and more particularly to a graphic display apparatus having a correction circuit that translates the X-Y coordinates of a work area on the data entry surface of the pad into the X-Y coordinates of a display area.
Known graphic display apparatus comprise a writing pad, a microprocessor-based controller and a display device. The writing pad has a data entry surface and generates a graphic signal indicating the X-Y coordinate position of a writing pen as it leaves a trace on the entry surface. The microprocessor-based controller responds to the graphic signal by generating digital X-Y coordinate signals to control the X-Y coordinate position of a cursor on the display area. It is advantageous to use a sheet of paper or a transparent plastic sheet to define a work area within the data entry surface of the pad to permit the pen to leave a trace on such permanent record mediums. However, the record mediums may differ in size and may be tilted with respect to the reference directions of the entry surface. Thus, the X-Y coordinate position of the writing pen differs from one record medium to another. Further it is desirable that the size and orientation of the displayed graphic be constant despite different work sizes and areas. This is particularly advantageous for transmitting graphic data between remotely located writing pads over telephone lines.
To overcome this problem, it has been proposed to construct a microprocessor-based controller by including in it a sequence of programmed instructions. During calibration modes, the pen is located at X-Y coordinate reference work positions and differences between these reference work positions are detected. Correction factors are calibrated in accordance with the detected differences and stored into respective memory locations. During data entry modes, the stored calibration data are retrieved from the memory. X-Y coordinate position indicating signals from the pad are corrected in accordance with the retrieved correction data through a number of logical operations involving additions, subtractions and multiplications. However, a sequence of such logical operations takes a substantial amount of time and hence it causes a substantial delay for the cursor to move from one point to another as the pen is rapidly moving on the writing pad. Further, if a plurality of writing pads are connected to a single controller, the memory capacity must be sufficient to store as many correction data as there are writing pads.